When exposed to radiation such as X-rays, an energy-storing phosphor (e.g., stimulable phosphor, which gives off stimulated emission) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating rays). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially scanning the storage panel with a stimulating rays such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and a phosphor layer provided thereon. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical damage.
The phosphor layer in the conventional storage panel generally comprises a binder and energy-storing phosphor particles dispersed therein.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. However, it is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of as high quality (in regard to sharpness and graininess) as possible.
For the purpose of improving the sensitivity and the image quality, it is proposed to form the phosphor layer by a gas phase-accumulation method such as vapor-deposition process or sputtering process. For example, in the vapor-deposition process, the phosphor or material thereof is heated with a resistance heater or an electron beam, vaporized and accumulated on a substrate (e.g., metal plate) to prepare a layer of the phosphor in the form of columnar crystals. The prepared phosphor layer consists of only the phosphor, and there are cracks among the columnar crystals. Accordingly, the stimulating rays can be applied efficiently enough and the emission can be collected also efficiently enough to improve the sensitivity. In addition, since the stimulating rays are kept from scattering horizontally, an image of high sharpness can be obtained.
JP-A-2001-324600 discloses a process for providing a protective layer on the phosphor layer formed by the gas phase-accumulation method. In the process, a solution of film-forming material is spread to coat the phosphor layer (in which there are many cracks extending in the thickness direction). JP-A-2001-324600 teaches that the film-forming material should not much soak into the cracks.
JP-B-63-25320 describes a protective film glued via an adhesive layer on the phosphor layer comprising binder and powdery phosphor. As examples of adhesive for the adhesive layer, a polyester adhesive is described. However, any detailed description on properties of the adhesive.
The applicant has already disclosed a radiation image storage panel comprising a stimulable phosphor layer, an adhesive layer, and a transparent moisture-proof film (Japanese Patent Application No. 2003-414265).
The transparent moisture-proof film comprises a transparent thin film having a thickness of 1 to 10 μm and a moisture-proof layer formed thereon, and is glued onto the phosphor layer via the adhesive layer. The adhesive layer disclosed in examples of the application is made of a conventionally used polyester adhesive (VYLON 300, Toyobo Co., Ltd.).
The present applicants have studied to find out an appropriate adhesive composition and note that, while the storage panel is used and/or stored for a long time, the adhesive layer gradually becomes thinner and finally almost disappears to lose adhesion strength if it is made of a resin having low glass transition temperature such as VYLON 300 (glass transition temperature (Tg):6° C.), and consequently that the protective film or the like (which is glued via the adhesive layer on the gas phase accumulation-made phosphor layer consisting of columnar crystals) partly separates (namely, partly comes off) from the phosphor layer to cause practical troubles (for example, to give an uneven radiation image). The applicants have further studied and finally found that the resin of a low glass transition temperature softens as time passes because it is affected by, for example, change of environmental temperature, and accordingly that the softened resin causes the capillary phenomenon to move into the cracks in the phosphor layer, so that the adhesive layer becomes thinner and finally almost vanishes.